diff --git a/mocha.js b/mocha.js
index f02d2a9563..d741dc9fcc 100644
--- a/mocha.js
+++ b/mocha.js
@@ -3069,6 +3069,7 @@ function clean(test) {
       title: test.title
     , fullTitle: test.fullTitle()
     , duration: test.duration
+    , err: test.err
   }
 }
 }); // module: reporters/json.js